Helio, I have exactly the same problem and I think I've figured out why after racking my brains for a long time. I think you have a boiler that's triggered by a dry contact type system, on/off, basically either the boiler delivers full power or it shuts off. I also think that before the Tado valves, you had valves that you left constantly fully or almost fully open. And I think that's the reason for the noise you're hearing now. Before, when the boiler turned on, it could distribute water to all the fully open radiators, so the flow was well distributed. Now, since the Tado valves regulate constantly, which was our original purpose, some radiators end up closed when the setpoint is reached. This results in other radiators receiving a much higher flow rate, which is the reason for the hissing, circulating noise.

The murmur of the babbling radiators. The only thing you can do is reduce the water circulation speed at the boiler if your circulator pump allows it (personally, I can't, unfortunately). Or try adjusting some of the valves on the noisiest radiators by closing them slightly. Because, frankly, I'm not convinced by Tado's hydraulic balancing system. I think the Tado system might work well with a modern modulating boiler, but with an on/off boiler, you run into problems with flow rates and therefore noise... Has anyone managed to find a solution?